The present invention relates to a method of manufacturing a semiconductor device, such as an IC (integrated circuit) or a discrete semiconductor device having an electrode thereon connected to a lead, and to semiconductor devices provided by implementation of the method.
FIG. 1 shows a method of wire bonding on such a semiconductor device. This wire bonding step serves to connect an aluminum electrode 3 (formed on a semiconductor chip 2) to a lead 4, using a gold capillary wire 1 which is supplied through a bonding tool 5. The lead 4 would typically be of a copper alloy which has received a surface treatment, such as plating with silver or the like.
To accomplish such a connection, an end of the gold wire 1 is made to melt by means of arc heat input, and is then solidified to form a ball 1a. Then the ball 1a is ball-bonded to the aluminum electrode (FIGS. 1a and 1b). After that, the wire 1 is led off and stitch-bonded to the lead 4 (FIGS. 1c and 1d). The thermosonic method is often employed for the bonding of the wire 1.
Since gold wire is expensive, and the long-term reliability of the junction between gold wire and the aluminum electrode is not sufficiently high, various alternative materials and bonding techniques have been studied.
One candidate to substitute for gold is copper, since copper is less expensive and offers good long-term reliability. A problem associated with copper is inferior bondability of copper wire to an aluminum electrode. If the power of the ultrasonic wave is increased in an attempt to assist the bond, aluminum may be driven outward, as indicated by reference numeral 3a in FIG. 2, so that the ball 10a of the copper wire 10 may abut the semiconductor chip 2. Thus, the electrode 3 and the semiconductor chip 2 may be damaged.
When this problem occurs, various known measures may be taken. For instance, the quality of the aluminum may be adjusted. But in view of the fact that the semiconductor devices are mostly mass-produced, it is necessary to be able to assess the bondability of the aluminum electrode film in order to maintain the desired quality without having bonding-related failures occur in manufactured product.
Conventionally, accurate assessment of the bondability requires various tests. These tests would include, e.g., a test of the bond strength, and observation of how the alloy layer is formed and how the aluminum film is deformed and driven away. (Such observations can be made by looking at the cross section of the bonded part.) The bond strength can be measured by measuring the shear strength of the bonded part. Typically, 40 grams-force (0.39 Newton) or more is a required level of the breaking load. For assessment of the electrode for the purpose of quality control during mass-production, a simpler test method is desired.
One known method of assessment of aluminum from the viewpoint of bondability is to measure the hardness (Knoop hardness). But this measurement is conducted under a situation different from the actual bonding. Moveover, the error of the measurement for a very light load is considerable. A very light load must be used for testing because the load under bonding is very light, and the measurement differs depending on the magnitude of the load.
Additional background information on conventional bonding methods may be found in U.S. Pat. No. 4,705,204; Kurtz, "Copper wire Ball Bonding," 34th ECC Proceedings (1984) at pages 1-6; and Onuki et al., "Study of Aluminum Ball Bonding for Semiconductors," 34th ECC Proceedings (1984) at pages 7-12; and in references cited in these publications; all of which are hereby incorporated by reference.